Phosphorus compounds are used in various fields generally as a multifunctional compound, and various kinds of compounds have been developed. The phosphorus compounds have excellent functions especially as a flame-retarder, and have been widely used. Resins to which the phosphorus compounds are to give flame retardancy are diverse, and examples of such resins include thermoplastic resins such as polycarbonate, an ABS resin, polyphenylene ether (PPE) and polyester (e.g., polyethylene terephthalate, polybutylene terephthalate); and thermosetting resins such as polyurethane, an epoxy resin and a phenol resin. Also, these resins are in such diverse forms as molded article and fiber article.
Flame retardancy of a resin depends generally on the phosphorus content in a phosphorus compound. However, addition of the phosphorus compound in too large an amount to a resin in an attempt to give the resin a desired degree of flame retardancy could greatly deteriorate properties of the resin itself, especially mechanical properties thereof. Therefore, in order to give a resin sufficient flame retardancy by addition of the phosphorus compounds in a smaller amount, there has been a demand for phosphorus compounds having a higher phosphorus content.
Also, there has been the demand that the phosphorus compounds to be added have a high stability even at high temperatures because resins are to be exposed to significantly high temperatures at steps of kneading and molding.
Structures of phosphorus compounds are classified mainly as phosphate, phosphonate, phosphinate, phosphite, phosphonite, phosphinite, phosphine oxide, phosphine and the like. Every phosphorus compound has at least one of these structures within one molecule, and some have two or more different structures.
A phosphate-phosphonate is an exemplary structure, and known are a phosphate-phosphonate containing a halogen atom such as chlorine, bromine or the like within one molecule, a phosphate-phosphonate containing an alcoholic hydroxyl group within one molecule, and a phosphate-phosphonate containing a lower alkyl group such as ethyl group.
There are various processes known as a process for preparing such phosphate-phosphonates.
For example, the specification of U.S. Pat. No. 4,697,030 describes a process for synthesizing a phosphate-phosphonate by reacting a phosphonate having an alcoholic hydroxyl group with either phosphorus oxychloride or phosphorus chloridate in coexistence of Lewis acid such as magnesium chloride as a catalyst and triethylamine as a hydrogen chloride scavenger.
However, there is a problem in this process in that the kind of a phosphonate usable a raw material is restricted. In order words, when a phosphonate having a primary hydroxyl group is used as a raw material, the reaction proceeds without causing any problem, as described in the Examples of the above specification. However, when a phosphonate having a secondary or tertiary hydroxyl group is used as a raw material, there arises a problem that the reactivity with a compound having a pentavalent P(═O)—Cl (phosphorus-chlorine) bond is extremely lowed, resulting in a reduced yield of the target compound.
Also, even when a strongly basic catalyst such as 4-dimethylaminopyridine or 1,8-diazabicyclo(5,4,0)undecene-7(DBU) is used in combination with triethylamine, the reactivity is not highly improved. Further, the above process, which uses the expensive catalysts, is not preferable in terms of cost. In addition, the use of these catalysts derives another problem of an increase in the amount of impurities prepared as a by-product.
Also, the specification of British Patent No. 941706 describes a process of synthesizing a trivalent phosphite compound with the use of, as raw materials,
trialkyl phosphite, a carbonyl compound such as a ketone or an aldehyde, and phosphorus chloridate. The phosphate-phosphonate can be obtained by oxidizing the trivalent phosphite compound prepared by the above process.
However, phosphite compounds have a defect that they will readily decompose under an acidic atmosphere. The synthesis process in the above specification is a synthesis under an acidic atmosphere carried out by adding the carbonyl compound to the phosphorus chloridate without using any catalyst, and thus has a problem that the product will decompose to reduce the yield.